Field of the Technology
The current invention relates to micromachined gyroscopes, in particular a toroidal ring gyroscope with a robust ring anchor and a distributed suspension system.
Description of the Prior Art
Coriolis Vibratory Gyroscopes (CVGs) can be divided into two broad categories based on the gyroscope's mechanical element: Degenerate mode gyroscopes which have x-y symmetry (Δf=0 Hz ideal) and non-degenerate mode gyroscopes which are designed intentionally to be asymmetric in x and y modes (Δf≠0 Hz). Degenerate mode CVGs have potential advantages over non-degenerate mode CVGs in terms of rate sensitivity, signal to noise ratio, power consumption, and potential to implement whole angle mechanization. However, mechanical elements with high-Q factor and very good frequency symmetry are required to utilize these advantages.
Realizing this potential, many MEMS degenerate mode gyroscopes have emerged in the recent years. For example, high aspect ratio ring gyroscopes have been demonstrated. A cylindrical rate integrating gyroscope with a Q-factor of ˜21,800 at 2.5 mm diameter has been demonstrated, along with a high frequency poly-silicon disk resonator gyroscope (DRG) with a Q-factor of ˜50 k at 264 kHz and 600 μm diameter was presented. Later a crystalline-silicon version of similar geometry was demonstrated with Q-factor ˜100 k. Q-factors as high as 1 million was demonstrated on a quadruple mass gyroscope (QMG), however the device had a 9 mm×9 mm footprint. Despite these successful implementations of degenerate mode operation, obtaining a high-Q factor in a compact volume remains to be a challenge due to factors such as support losses, thermo-elastic dissipation, and viscous damping.
Axi-Symmetric MEMS Gyroscopes Operating in Wineglass Modes
Axi-symmetric MEMS gyroscopes fabricated using lithography and deep reactive etching (DRIE) of silicon are well known. These devices typically consist of extruded 2-D geometries (rings, disks, concentric rings etc.), that operate on n=2 and n=3 wineglass modes.
One such device is a micro-machined angle-measuring gyroscope. The design employs multiple concentric ring structures and additional electrodes in the form of parallel plates to increase the effective mass (hence potentially the Q-factor) of the resonator and the total available capacitance of the drive and sense modes. Designs that employ central anchor points as well as outer anchors have also been proposed. A control algorithm for whole-angle operation have also been included, complete with parametric drive and quadrature compensation loops. Parametric excitation of oscillations is the excitation of oscillations in an oscillatory system through the periodic variation of the value of one of the system's oscillation parameters, that is the parameters on whose values the system's potential energy, kinetic energy, and period of natural oscillations essentially depend.
Another such iteration have been poly-silicon ring gyroscopes using the high aspect-ratio combined poly and single-crystal silicon process (HARPSS). The HARPSS process can create very high aspect ratio capacitive gaps (as low as 50 nm) using a sacrificial SiO2 layer and PECVD poly-silicon on side walls of the capacitive gaps. The main body of the gyroscope consists of a ring structure that is supported by support springs and a center anchor. The ring gyro had 1.1 mm outer diameter and a device layer thickness of 80 μm with a 120 μm diameter central post and 1.4 μm capacitive gaps. A large frequency split was observed between the two degenerate n=2 wineglass modes, which is associated to the cos(4Θ) modulus of elasticity dependence of single crystalline silicon, thickness modulation along the perimeter of the ring was proposed to combat this effect. Electrostatic testing revealed a Q-factor of 20000 at 27.3 kHz. Minimum detectable signal was measured as 0.04 deg/sec at 10 Hz bandwidth, limited by the noise in the interface electronics.
Bulk acoustic wave silicon disk gyroscopes have also been previously demonstrated. As opposed to most flexural type gyroscopes the bulk acoustic wave disk gyroscopes operate in the MHz range. Lower amplitude of motion due to the higher frequency and stiffness of the resonant modes is compensated by the use of HARPSS process to create extremely small capacitive gaps of 250 nm. Disk gyroscopes with a diameter of 800 μm were fabricated on a 50 μm single crystal silicon device layer. Electrostatic frequency sweeps of the n=3 mode revealed a Q-factor of approximately 200000 at 5.9 MHz. A bias stability of 0.175°/s was obtained at approximately 2.5 s through Root Allan Variance Analysis.
Another example in the prior art is a single-crystal-silicon cylindrical rate integrated gyroscope (CING) using a silicon on glass (SOG) process. The device consists of concentric silicon cylinders (rings) that are connected to each other through a silicon back-plate, providing an order of magnitude increase in effective proof mass over a ring gyroscope. Electrostatic testing of a 2.5 mm radius LING revealed Q-factors on the order of 20000 at 17.9 kHz with a frequency split (Δf) of approximately 10 Hz or relative frequency split of 0.58% (Δfn=2/fn=2). A 6 mm radius version was later fabricated with Q-factors up to about 100000. The gyroscope was later mode-matched and operated in whole-angle mode using a commercial defined radio hardware. Root ALLAN variance analysis showed a bias stability of 153_=s. However it was later found out that the gyroscope has very low angle gain of 0.011 due to the fact that majority of the kinetic energy is stored in the out of plane mode.
Using a similar approach, a poly-silicon disk resonator gyroscope (DRG) have been demonstrated. The device consists of multiple concentric rings that act both as the proof mass and a distributed spring structure. The DRG in this work had 0.6 mm diameter with a 20 μm device layer and 1.5 μm capacitive gaps. A wafer-level epitaxial polysilicon encapsulation process was used to vacuum package the gyroscope during fabrication. A Q-factor of approximately 60000 was observed at about 264 kHz. Rate mode operation using the n=2 wineglass mode showed a bias stability of 3.29°/hr at 292 s.